i2c: mpc: cleanup clock API use

make the MPC I2C driver get, prepare and enable the gated clock item for
register access during probe; disable and unprepare the clock upon
remove(), put is done by the devm approach; hold a reference to the
clock over the period of use

clock lookup is non-fatal in this implementation as not all platforms
may provide clock specs in their device tree, but failure to enable a
clock when specified is considered fatal

Signed-off-by: Gerhard Sittig <gsi@denx.de>
Signed-off-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>